1. Field of the Invention
The present invention relates to a method of detecting recovery from fault in a data transmission system which effects loopback control, and particularly to a method of detecting recovery from a fault in a looped data transmission system which is equipped with looped transmission lines of two systems having different transmission directions, a plurality of stations and at least one control station connected to the transmission lines of these two systems, and which establishes the loopback configuration when the transmission lines of the two systems become defective over the same section, so that stations adjacent to the faulty section send signals from one system back to the other system.
2. Description of the Prior Art
In case transmission lines of two systems become faulty, the signals are sent back to the control station by two stations that are adjacent to the faulty section to perform a so-called loopback control, in order to continue the operation by cutting off the faulty section, as has been disclosed, for example, in Japanese Patent Application Laid-Open No. 79552/1981.
When the fault in the loop transmission lines is recovered by maintenance and checking, on the other hand, it is essential for the transmission system to recognize the recovery as quickly as possible to reconfigure the network.
A method of detecting the recovery of a fault that is suited to such a situation has been taught, for example, in Japanese Patent Application Laid-Open No. 50639/1984. According to this method of detecting the recovery of a fault, the two stations (hereinafter referred to as loopback end stations) adjacent to the faulty section send supervisory signals to each other via the faulty section, and supervise the supervisory signals sent via the faulty section. When at least one of the stations has received a supervisory signal, it is judged that the section of the system of the receiving side has recovered from the fault.
With this method of detecting the recovery of fault, the recovery of fault can be detected without inconvenience when the fault occurs at one place only. When the fault occurs at a plurality of places, however, this method is not capable of detecting the recovery of the fault until the faulty portions are all recovered.
That is, even when the faulty sections are partially recovered, a supervisory signal sent for one loopback end station does not reach another loopback end station since it is interrupted by a faulty section that has not been recovered.
By employing such a method of detecting the recovery of a fault, the stations which are once separated from the transmission system are not allowed to participate in the data transmission, since they are maintained separated from the transmission system until the faulty sections are all recovered.